1. Field of the Invention
The present invention relates to an optical scan apparatus which scans a scanning plane with a light beam, an image formation apparatus including the optical scan apparatus, a manufacturing method for an optical deflector used in the optical scan apparatus, a processing method for a deflection surface of a polygon mirror of the optical deflector, and a polygon mirror processing apparatus.
2. Description of Related Art
A known image formation apparatus using the Carlson process forms images by, for example, scanning the surface of a rotating photoconductive drum with a plurality of light beams to form an electrostatic latent image thereon, visualizing the electrostatic latent image as a toner image, and fusing the toner image on a sheet of paper as a recording medium. In recent years, this type of image formation apparatus has been well used in easy printing as an on-demand print system. There has been increasing demand for formation of high-density images and speeding-up of image outputs.
In general, in order to speed up the image outputs, a print speed is increased by increasing the rotation speed of a polygon mirror deflecting the light beam and the rotation speed of the photoconductive drum. However, there is a problem that along with the increase of the rotation speed of the polygon mirror, noise and vibration from a drive system is increased as well as an amount of heat emission. Further, the speeding-up of the image outputs has a trade-off relation with formation of the high-density images so that image quality deteriorates as the rotation speed of the polygon mirror increases.
In view of the above problems, Japanese Patent No. 3227226 has disclosed an image formation apparatus which includes a multi-beam light source and scans the photoconductive drum with plural light beams in order to concurrently realize the formation of the high-density images and the speeding-up of image outputs. This image formation apparatus is provided with a vertical cavity surface emitting laser (VCSEL) having plural luminous points and can collectively deflect the plural light beams from the VCSEL by the polygon mirror to thereby scan the photoconductive drum with the plural light beams at the same time.
Also, with regard to the speeding-up of the operation of a multi-color image formation apparatus, Japanese Laid-Open Patent Application Publication No. 2005-92129 has disclosed an optical deflector with a plurality of small-sized polygon mirrors which are superimposedly disposed in a rotation axis direction such that respective deflection surfaces of the polygon mirrors are shifted by a predetermined angle (with a phase difference), and coupled firmly with each other. With such a configuration, the optical deflector generates little noise even during the high-speed rotation, and the polygon mirrors are not likely to be shifted from each other in accordance with angular velocity changes or temperature changes. Accordingly, such an optical deflector does not cause color shifts in images. Combining the optical deflector with such polygon mirrors with the afore-mentioned vertical cavity surface emitting laser makes it possible to realize a multi-color image formation apparatus which can form high-density images with less color shifts at high speed.
However, there is a problem that in deflecting multiple light beams from a light source with a conventional optical deflector, spots of the light beams have different shapes and sizes due to the shapes of deflection surfaces of the optical deflector, causing generation of scattered light. Specifically, when the deflection surface has an undulation (unevenness) around incident positions of the light beams, the spot shapes of the light beams are differentiated due to the lens effect, resulting in deteriorating granularity of the images. Particularly, in the multi-color image formation apparatus forming images by superimposing toner images of plural colors, dot shifts occur in every color, and the image quality as color reproducibility is degraded.
Generally, the deflection surfaces of the optical deflector are formed by mirror-cutting; therefore, there are minute cutting traces left thereon. In the optical scan apparatus with the multiple light beams whose deflection area is broader than that of a single light beam, striking the cutting traces, the light beams will be scattered, which may cause generation of a ghost image.
Moreover, from the viewpoints of cutting process for the deflection surface, Japanese Laid-Open Patent Application Publication No. 2001-228432 has disclosed a mirror-cutting process in which polygon mirrors and a motor are integrated as a rotary body and the rotary body is rotated to mirror-cut the polygon mirrors. In this case, the outer diameter of the polygon mirrors is the greatest in the rotary body, and vertical two-stage mirrors are disposed at the same phase in the rotation direction. Because of this, it is easy to mirror-cut the polygon mirrors without interference of the rotary body and a cutting tool.
Japanese Laid-Open Patent Application Publication No. 2001-322012 has disclosed a polygon mirror processing device which includes a rough cutting tool 1a and a finish cutting tool 1b both attached to a holder in such a manner that the cutting tool 1a comes ahead of the cutting tool 1b in a feeding direction of a work (polygon mirror), and performs the rough cutting and finish cutting to the polygon mirror at the same time.
It is difficult to mirror-out the polygon mirrors with use of the above known processing device when the polygon mirrors are superimposedly disposed and fixed in a rotation axis direction, having deflection surfaces with a phase difference in a rotation direction, since there is a possibility that a portion thereof not being processed interferes with the cutting tools during the cutting process. To mirror-cut such polygon mirrors, they have to be individually cut first and then superimposedly disposed. However, there is a problem that when disposing the polygon mirrors, the fixed positions thereof may be shifted from each other, causing rotational movements or deformation of shape of the deflection surfaces and degrading surface precision. Further, in the optical scan apparatus and the image formation apparatus, the superimposed disposition causes variations of the deflection surfaces in the inclination direction relative to the rotation axis, resulting in deterioration in print quality.